1. Field of the Disclosure
The present disclosure relates to a surface emitting laser having a plurality of quantum wells, and more particularly, to a high-power surface emitting laser capable of recycling electrons and holes by inserting a tunnel junction between quantum wells.
2. Description of the Related Art
A vertical external cavity surface emitting laser (VECSEL) typically includes a substrate, a lower distributed Bragg reflector (DBR) layer, an active layer, and an upper DBR layer. Light generated in the active layer is repeatedly reflected back and forth between the upper and lower DBR layers. Light amplified in the active layer is output as a laser beam.
As research into fabrication of laser-based televisions using a laser has recently received more attention, the demand for compact high-power lasers with output power greater than several watts increases. In a surface emitting laser, such as a VESCEL or VCSEL, a gain region should be large to provide high output power, and current injected into the active layer must have a uniform spatial distribution to provide single transverse mode oscillation.
However, when an aperture area is increased to provide high output power, current density is high at the edges of an active layer but is low at the center because current flows more easily through the edges of the active layer where the resistance is low. The uneven distribution of current density makes it difficult to achieve single transverse mode oscillation.
Meanwhile, increasing the size of the gain region can be achieved by increasing the number of quantum wells without increasing the aperture area. FIG. 1 is a cross-sectional view of a multi-quantum well (MQW) structure for a conventional surface emitting laser. Referring to FIG. 1, the conventional surface emitting laser includes first and second DBR layers 31 and 36 respectively having a multi-layer structure, a p-cladding layer 32 and an n-cladding layer 35 located to face the first and second DBR layers 31 and 36, respectively, a plurality of barriers 33, and a plurality of quantum wells 34 alternately sandwiched between the plurality of barriers 33. The p-cladding layer 32 and the n-cladding layer 35 respectively provide holes and electrons to the quantum wells 34 where the electrons and the holes recombine to generate light.
The first and second DBR layers 31 and 36 bounce back and forth the light generated in the quantum wells 34, forming a standing wave. To maximize laser oscillation efficiency, the quantum wells 34 should be located at an anti-node (the point of maximum displacement) of the standing wave. As the number of quantum wells increases by one, a cavity length between the first and second DBR layers 31 and 36 increases by a ½ wavelength. When the entire length of cavity is 4 to 5 times greater than the wavelength, electric resistance is excessively high since the barriers 33 between the quantum wells 34 are undoped, resulting in a sharp decrease in emission efficiency. For a laser optically pumped by a pump laser, increasing the number of quantum wells suffers no degradation in emission efficiency. Conversely, for an electrically pumped laser, increasing the number of quantum wells results in degradation of emission efficiency.